1. Field of the Invention
The present invention relates to a multi-beam scanning apparatus used for a laser beam printer, digital copying machine, and the like.
2. Related Background Art
In recent years, multi-beam scanning apparatuses for simultaneously writing a plurality of lines using a plurality of laser beams are being developed in electrophotographic apparatuses such as a laser beam printer.
The multi-beam scanning apparatus simultaneously scans a plurality of laser beams apart from each other. As shown in FIG. 1, in the multi-beam scanning apparatus, a multi-beam semiconductor laser 111 serving as a light source for a multi-beam light source unit 101 emits two laser beams P1 and P2. The laser beams P1 and P2 are collimated by a collimator lens 112, irradiate a reflecting surface 103a of a rotary polygon mirror 103 via a cylindrical lens 102, and form an image on a photosensitive member on a rotary drum 105 via an imaging lens 104.
The two laser beams P1 and P2 are incident on the reflecting surface 103a of the rotary polygon mirror 103, scanned in the main scanning direction, and form an electrostatic latent image on the photosensitive member along with main scanning by rotation of the rotary polygon mirror 103 and subscanning by rotation of the rotary drum 105.
The cylindrical lens 102 linearly focuses the laser beams P1 and P2 on the reflecting surface 103a of the rotary polygon mirror 103. The cylindrical lens 102 has a function of preventing a point image formed on the photosensitive member in the above manner from being distorted due to surface tilt of the rotary polygon mirror 103. The imaging lens 104 is made up of a spherical lens and toric lens. The imaging lens 104 has a function of preventing distortion of a point image on the photosensitive member, similar to the cylindrical lens 102, and a correction function of scanning the point image on the photosensitive member in the main scanning direction at a constant speed.
The two laser beams P1 and P2 are respectively split by a detection mirror 106 at the end of the main scanning plane (X-Y plane), guided to a photosensor 107 on an opposite side to the main scanning plane, and converted into write start signals in a controller (not shown) to be transmitted to the multi-beam semiconductor laser 111. The multi-beam semiconductor laser 111 receives the write start signals to start write modulation of the two laser beams P1 and P2.
By adjusting the write modulation timings of the two laser beams P1 and P2, the write start (write) position of an electrostatic latent image formed on the photosensitive member on the rotary drum 105 is controlled.
The cylindrical lens 102, rotary polygon mirror 103, imaging lens 104, and the like are mounted on the bottom wall of an optical box 108. After the respective optical components are mounted in the optical box 108, the upper opening of the optical box 108 is closed with a lid (not shown).
As described above, the multi-beam semiconductor laser 111 simultaneously emits the laser beams P1 and P2. The multi-beam semiconductor laser 111 is integrated via a laser holder 111a with a lens barrel 112a incorporating the collimator lens 112, and the integral unit is mounted on a sidewall 108a of the optical box 108 together with a laser driving circuit board 113.
In mounting the multi-beam light source unit 101, the laser holder 111a holding the multi-beam semiconductor laser 111 is inserted into an opening 108b formed in the sidewall 108a of the optical box 108. The laser holder 111a is fitted in the lens barrel 112a of the collimator lens 112, the focal point and optical axis of the collimator lens 112 are adjusted, and the lens barrel 112a is adhered to the laser holder 111a. As shown in FIG. 2A, the laser holder 111a is rotated through a predetermined angle θ to adjust a straight line connecting the emission points of the laser beams P1 and P2, i.e., the inclination angle of a laser array N. More specifically, as shown in FIG. 2B, the beam interval between the laser beams P1 and P2 emitted by the multi-beam semiconductor laser 111 is adjusted to make a pitch S between imaging points A1 and A2 on the rotary drum 105 in the main scanning direction, and a pitch, i.e., line interval T in the subscanning direction coincide with design values. After this adjustment, the laser holder 111a is fixed to the sidewall 108a of the optical box 108 with a screw or the like.
In the prior art, however, when the multi-beam light source unit is to be fixed to the optical box, the whole multi-beam light source unit is rotated through the predetermined angle θ together with the laser driving circuit board, thereby obtaining the line interval T. To realize this, a space enough to rotate the large-area laser driving circuit board must be prepared outside the optical box, which interferes with downsizing of the whole apparatus.
Further, an error allowable value for adjustment of the line interval T is as strict as several μm or less. If the angular adjustment range in assembling the multi-beam light source unit to the optical box is wide, high-precision adjustment is difficult to complete within a short time. The multi-beam light source unit cannot be assembled with high working efficiency and high reliability.